Blood is typically collected from patients in closed, evacuated sample tubes. The samples contained in these tubes are later aspirated from the tubes so that various tests may be performed. However, in order to obtain a substantially homogenous mixture for testing (e.g., an even distribution of blood cells) it is first necessary to mix the sample before aspiration. Further, because vigorous mixing may cause damage to delicate blood cells, a means for gently mixing the samples before aspiration is required.
One system for mixing and aspirating blood samples is disclosed in U.S. Pat. No. 4,311,484, wherein a pair of vertical screw-threaded shafts convey a series of closed containers downward towards a retractable aspiration needle. Differential speed of the feed screws imparts a gentle rocking and rotation on the tubes as they are transported towards the aspiration needle. The disclosed system is limited, however, in that sample tubes must be input individually and optimum mixing is not adequately accomplished. In addition, only tubes of like dimensions may be processed simultaneously. As various tube manufacturers produce sample tubes with different dimensions and sizes, it is desirable that an automated sampler be capable of dealing with sample tubes of varying characteristics simultaneously.
Another system, disclosed in U.S. Pat. No. 4,475,411, includes a vertically mounted, rotatable round cassette in which sample tubes are mounted along the periphery. The number of tubes is limited in this system by the size of the cassette. In addition, the cassette cannot be changed while samples are being aspirated.
U.S. Pat. No. 4,609,017 describes a system in which sample tubes are housed in racks, which are vertically stacked, with the tubes in their horizontal orientation, in an input compartment. The racks are stripped from the bottom of the stack one at a time by an input elevator and lowered onto a combined conveyor belt and tilt table. The tilt table mixes the tubes as the conveyor advances the racks to an aspiration station. The sample tubes are then individually indexed to an aspiration probe with the tilt table tilted so that the sealed tube end is below the opposite end. Once samples have been taken from every tube in the rack, the rack is advanced to an output elevator and stacked vertically in an output compartment.
The racks used in the above system are described in U.S. Pat. No. 4,534,465. Each rack includes a base, a front wall and an intermediate wall, wherein the intermediate wall is disposed approximately midway between the base and the front wall. The front wall and the intermediate wall include a linear series of aligned apertures for receiving sample tubes. A spring is disposed between the front wall and the intermediate wall for holding sample tubes against the upper edges of the apertures.
The system of the '017 patent (including the rack of the '465 patent) is limited, however, in the ability to add or remove racks from the input and output stacks. Because the racks are vertically stacked in the input and output compartments, the ability to add or remove racks from the middle of these stacks is precluded without first removing racks above the desired position.